1. Field of the Invention
This invention relates to a color copying machine or an electrophotographic copier.
2. Prior Art
A conventional color copying machine generally comprises a light source, movable mirrors, a photoconductive drum and a transfer drum, and these component parts are accurately driven in accordance with a predetermined timing to form an image on a record sheet.
FIG. 1 is a diagrammatical illustration showing the construction of a conventional color copying machine. The color copying machine comprises a body 1 which is provided at its upper portion with a platen 2 for placing a manuscript thereon. Mounted on the body 1 below the platen 2 is a scanning unit 3 which includes an elongated lamp 4, first and second mirrors 5 and 6, a filter-and-lens unit 7 and third and fourth mirrors 8 and 9. The lamp 4 is mounted on the body 1 so as to be reciprocally movable in unison with the first mirror 5, as indicated by arrows A and B in FIG. 1. The second mirror 6 is arranged so as to move in accordance with movement of the lamp 4 and first mirror 5 at a speed which is half of the speed of the movement of the lamp 4 and first mirror 5.
In operation, the lamp 4 and the first mirror 5 are first moved in the direction indicated by the arrow. A. As a result, an outer peripheral surface of the photoconductive drum 11 rotating counterclockwise is exposed to a beam of light which represents the image on the manuscript placed on the platen 2. In this case, the filter-and-lens unit 7 has been changed over so that it passes components of the light other than the yellow component. Also, the photoconductive drum 11 has been electrically charged by the charging device 12. And therefore, an electrostatic latent image corresponding to the yellow component of the image on the manuscript is formed on the surface of the photoconductive drum 11. A yellow toner is then attached to the electrostatic latent image on the photoconductive drum 11 by a first developing device 13, so that a yellow toner image is formed on the peripheral surface of the photoconductive drum 11.
On the other hand, a record sheet fed from a record-sheet tray 14 is wrapped around a transfer drum 15 rotating clockwise, and fed to a transfer position where the peripheral surface of the transfer drum 15 most closes with the peripheral surface of the photoconductive drum 11. As a result, the yellow toner image on the photoconductive drum 11 is transferred to the record sheet on the transfer drum 15. As the transfer of the toner image proceeds, the portion of the peripheral surface of the photoconductive drum 11 from which the toner image has been transferred to the record sheet is cleaned by a cleaning device 16.
When the transfer of the entire yellow toner image to the record sheet is completed, the filter-and-lens unit 7 is changed over so that it passes color components of the light other than the magenta component. At the same time, a second developing device 17 for a magenta color is selected, and another transfer operation is carried out in a manner described for the yellow toner image. Then, the filter-and-lens unit 7 is again changed over so that it passes color components of the light other than the cyan component, and a third developing device 18 is selected. And, a further transfer operation is carried out in the same manner. Thus, the toner images of the three primary colors, namely, yellow, magenta and cyan, are combined on the surface of the record sheet wrapped around the transfer drum 15 to form a color image. The record sheet on the transfer drum 15 is then fed by an endless belt 21 to a fixing device 22 at which the color image formed on the record sheet is fixed to the record sheet. When the fixing operation is completed, the record sheet is discharged to a tray 23, and the copying operation is terminated.
FIG. 2 shows in more detail the structure of the scanning unit 3, photoconductive drum 11 and transfer drum 15. A drive force of an electric motor (not shown) is transmitted through a chain 31 to a sprocket 33 which is mounted on a shaft 32 of the photoconductive drum 11. The shaft 32 is provided, at one end of the photoconductive drum 11, with a gear 34 which is in mesh with another gear 36 mounted on a shaft 35 of the transfer drum 15. With this arrangement, when the sprocket 33 is driven, the photoconductive drum 11 and the gear 34 rotate, so that the shaft 35 is driven through the gear 36, whereby the transfer drum 15 is rotated. In this case, the gears 34 and 36 are so arranged to have the same diameter and pitch, so that the photoconductive and transfer drums 11 and 15 rotate in the opposite directions at the same rotation speed in synchronization with each other. The transfer drum 15 is provided with retractable projections 37 for defining the position of a record sheet wrapped therearound so that the leading edge of each record sheet comes to the same position on the transfer drum 15.
On the other hand, the shaft 32 has a pulley 42 rotatably mounted thereon through a bearing 41. This pulley 42 is provided with a hook (not shown) driven by a drive means such as a solenoid. When the hook is driven so as to engage with a pin 43 mounted on the sprocket 33, the rotation of the shaft 32 is transmitted to the pulley 42, so that the pulley 42 rotates in unison with the photoconductive drum 11. The rotation of the pulley 42 is transmitted through a wire 44 to another pulley 48 of which rotation is transmitted through a shaft, pulleys, wires and so on to the scanning unit 3. Thus, when the pulley 42 is rotated, the lamp 4 and its associated parts move in the direction indicated by an arrow A in FIG. 2 in accordance with the rotation of the photoconductive drum 11. When the hook is released from the pin 43, the lamp 4 is returned to its home position by means of a spring (not shown).
With the above-described conventional color copying machine, the scanning unit 3 and the photoconductive drum 11 are mechanically connected to each other so that electrostatic images are formed on the photoconductive drum 11 at the same place. Also, since the photoconductive drum 11 and the transfer drum 15 rotate in opposite directions and the position of the leading edge of each record sheet wrapped around the transfer drum 15 is constant, positions of toner images transferred to a record sheet substantially coincide with one another, whereby a color image is formed.
To avoid undesirable shift or displacement of toner images transferred onto a record sheet, which deteriorates the quality of the resultant color image, the relationship between the position of the scanning unit 3 and that of the photoconductive drum 11 and the relationship between the position of the photoconductive drum 11 and that of the transfer drum 15 must be controlled extremely accurately. With the structure of the above-described conventional copying machine, however, all of the movable sections, i.e., the scanning unit 3, photoconductive drum 11 and transfer drum 15, are mechanically connected so as to move cooperatively with one another, so that the initial or home positions of those movable sections may vary due to their aged deterioration. Thus, the conventional color copying machine is deficient in that toner images transferred to a record sheet are liable to shift from one another.
With the conventional color copying machine, when it is desired to produce a copy of the original at a magnification rate other than "1", pulleys (for example, pulleys 48a and 48b) of different diameters must be provided, as shown in FIG. 2. In this case, one of the pulleys 48, 48a and 48b is selected in accordance with the magnification rate to change the speed of the scanning unit 3. With this structure, however, to increase the number of magnification rates available, the number of pulleys must also be increased. This makes the transmission mechanism complicated and requires much space.